The invention relates to a method for starting a casting operation in a two-roll casting device without the use of a start-up strand, and to an apparatus for carrying out this method.
Chilled molds with a continuous mold cavity, in which the metal melt that is introduced on the entry side solidifies at least in the region of contact with the mold cavity walls, are mainly used for the production of a continuously cast metal strand of indeterminate length. A substantially fully solidified metal strand is taken out of the mold on the exit side. When starting the casting operation, the mold cavity first of all has to be filled with metal melt, and in particular, with a predominantly vertical orientation of the mold cavity, a starting piece which has been completely solidified has to be obtained so that the metal melt does not flow through the mold in uncontrolled fashion and escape from it. In this context, in particular the casting thickness of the metal strand that is to be produced, the solidification conditions and the quantity of heat that is to be dissipated through the mold cavity walls during the short residence time in the mold are of considerable importance.
To reliably avoid uncontrolled escape of metal melt from the mold during the starting phase of the casting process, it is customary to use a start-up strand to be introduced into the mold before casting commences, the start-up strand substantially but not necessarily completely closes up the exit cross section of the mold cavity and is only discharged from the mold by means of a pair of driving rolls once a solid join has been formed between the introduced melt and the start-up strand head and a pronounced strand shell of sufficient thickness along the mold cavity walls. This start-up operation requires at least one new start-up strand head to be coupled to the start-up strand each time the casting installation is restarted. A start-up strand of this type, as is used in the case of strip steel casting molds formed by wide side walls and narrow side walls, is known, for example, from U.S. Pat. No. 4,719,960.
A start-up strand specifically for use in a two-roll casting installation is described in EP-A 208 642. This start-up strand includes a start-up head with two flanges formed by thin strips of sheet metal which bear against the lateral surfaces of the casting rolls and thereby form a space for receiving the metal melt flowing in. The start-up strand and the strip which is initially cast are discharged from the casting nip formed by the casting rolls immediately after the first strand shell has been formed.
At very small casting thicknesses, preferably below a casting thickness of 5.0 mm, a start-up strand is not necessarily required, since the rapid solidification of the metal melt at the mold walls means that the open casting nip is bridged within a very short time. A number of start-up methods which do not require a start-up strand are likewise already known.
By way of example, JP-A 61 266 159 has disclosed a starting method in which the two interacting casting rolls, prior to the start of casting, are moved into a starting position in which there is no casting nip and the casting rolls are stationary. Immediately after the melt starts to be supplied and the first strand shell has formed on the two lateral surfaces of the casting rolls, the latter are moved apart to the operating casting nip (strip thickness), and the casting velocity is increased to the operating casting velocity along a run-up curve. However, a starting operation with stationary casting rolls is very unreliable, since the actual casting level in the melt space cannot be measured with the required accuracy all the way to the narrowest cross section between the casting rolls. Therefore, neither an increase in force between the two casting rolls nor the degree of filling of the mold can be suitably controlled. A different level of solidification of the melt along the strip width and in particular in the vicinity of the side plates can cause considerable wedge formation resulting from solidified metal above the narrowest cross section and can then lead to damage to the side plates. Furthermore, with a starting method with stationary casting rolls of this type, there is an increased risk of strand shell stickers forming on parts of the lateral surface of the casting rolls.
WO 01/21342 has disclosed an initial casting method for a two-roll casting device, in which before the supply of melt commences the casting nip between the two casting rolls is set to a value which is reduced compared to the operating casting nip. The melt is supplied with the casting rolls rotating, with the casting velocity being set in such a way that the thickness of the strip produced is greater than the casting nip that has previously been set. In principle, a reduced casting nip reduces the tendency of metal melt to drip through. On the other hand, small casting nips result to an increasing extent in the drawbacks which have been described above with regard to JP-A 61-266 159, in particular the probability of damage to the side plates.
Further initial casting methods for standard two-roll casting devices with special method stipulations for the casting velocity in the starting phase or the selection of a suitable starting casting thickness in relation to the operating casting thickness are already known from JP-A 63-290654, JP-A 1-133644 or JP-A-6-114504. EP-A 867 244 describes a control means by which in the starting phase of the casting process, in successive time periods, first of all the instantaneous velocity of the casting rolls is controlled as a function of a bath level measurement in the melt pool between the casting rolls, and then the supply of metal melt is controlled as a function of a roll velocity measurement.
Therefore, it is an object of the present invention to avoid the drawbacks of the prior art described in the introduction and to propose a method for starting a casting operation in a two-roll casting device and a device for carrying out the method, in which it is possible to keep the passage of metal melt through the casting nip at a low level and at the same time the likelihood of wedges and thickened portions forming at the start of the cast strip is as far as possible avoided. At the same time, it is intended for a first piece of the cast strip, which does not meet the quality demands imposed for continuous production, to be separated from the strip which is produced subsequently under substantially steady-state operating conditions without mechanical separating devices being required for this purpose.